The findings of this thesis present a clear indication of the importance of bioavailability when modelling metal toxicity in soil. While the bioavailability models were developed on the basis of toxicity experiments performed in different soils covering a wide range of soil properties, the future use of these models may be limited to the range in soil properties for which they were derived. Nevertheless, we have reasons to be optimistic about regulatory
application of the developed bioavailability models since they provide a mechanistic framework and can be easily modified to incorporate the effects of other toxicity-modifying factors. A series of validation studies and meta-analysis using our model concepts are encouraged to be done in future. Besides, it would be possible to apply our models to geographically oriented databases in order to do GIS-mapping of the site-specific risks of metals on a large scale, as long as the basic soil information (i.e., porewater chemistry) required by the models is available.
Various single-species screening tests are often employed to detect possible harmful effects of metals in soil. In this thesis, earthworms were selected as the test species as they are the dominant soil macrofauna and play a vital role in soil functioning. However, numerous species and a complex food web are exposed in the soil environment. Therefore, a battery of toxicity tests should be used to evaluate the effects on different trophic levels, as well as acute, chronic, and next-generation effects. Until now, very few research efforts have been made to determine the minimum battery of tests needed. To obtain a balanced battery of tests, it is recommended that a series of important criteria for selecting representative species among others need to be met, such as having different life-histories and exposure routes, and belonging to different functional and taxonomic groups. Besides, the selection of a battery of tests should be tailored to specific protection objectives.
Most of the bioavailability models are developed at a fixed time point, while the toxicodynamic part of the model is simply to relate the bioavailable faction (e.g., f value in BLM) to a toxicological endpoint (e.g., LC50 or EC50). This is also the case in our study. However, it has been recognized that bioavailability is not a static but a dynamic phenomenon (see Figure 1.1). For a better understanding of metal bioavailability, the underlying toxicokinetic and toxicodynamic processes deserve further investigation. This kind of research will enable the extrapolation of metal effects in the course of time and from soil organisms to higher organisms.
Our study distinguished the interactions of Cd and Zn at the exposure level from those at the organism level. However, the interactions of metal mixtures observed in this study yield little information about the internal processes involved. In fact, mechanistic pathways of metals inside the organism are poorly known. We therefore recommend that future studies should investigate the mechanisms of mixture interaction and identify the principles of combined toxicity for developing predictive models. Potential topics include, for example, considering the distribution and detoxification mechanisms (i.e., toxicological bioavailability) of one metal in the presence of other metals, and linking mixture effects to the expressions of specific proteins or genes (proteomics and genomics tools).
The issues of metal bioavailability and mixture toxicity have increasingly gained attention on the agenda of risk assessors and risk managers. The findings in this thesis therefore well fit into the broader picture of soil metal risk assessment. Despite the lack of mechanistic understanding of internal processes, the awareness of bioavailability and mixture toxicity has triggered the development of a series of chemical and modelling approaches to normalize the toxicity data in different environmental conditions, and thus has greatly improved risk assessment and standard setting. In case of remediation, environmental regulators are able to choose the sites for remediation where the actual risks are greater rather than blindly choosing the polluted localities where the total metal concentrations are
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relatively high. This also avoids unnecessarily strict requirements for cleanup and thus avoids expenditure of funds that could be better used to remediate additional areas. We recommend continuing to discuss and question the relevance of the current approaches for risk assessment and decision making, and to update them according to the available tools and scientific knowledge on bioavailability and mixture toxicity.
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